There are some files on my ext4 formatted SD card which have been overwritten with files of the same filename.

There is plenty of space left on the SD card. So I thought, the new written data should have been written to previously free and unused sectors on the SD card due to flash memory wear leveling. However, PhotoRec cannot find the overwritten files - only the current files.

I couldn't find out, if the files are unrecoverable, because the overwriting has written onto the same sectors - or if there is a chance to recover the files, be it due to the design of the ext4 file-system or due to flash memory wear leveling or something else.

I tried to describe my problem as extensive as possible. If I could add something else to clarify the issue, please tell me.

Wear leveling is transparent to the OS point of view. If the same sectors have been used to store a file, the content of the previous file that was sitting at this location is lost.
There is no way to access old versions of a sector without bypassing the flash controller, very few data recovery companies can do it.